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“Similarity and contagion” is a blanket term used to refer to ways of thinking that sees similar objects as metaphysically connected (similarity) and objects that come into contact once as always being in contact (contagion). The term is used in cognitive psychology and anthropology for these universal tendencies in human reasoning. Anthropologists have done surveys of hundreds of cultures; including primitive tribal cultures essentially unchanged for thousands of years, and have found similarity and contagion to be completely universal.
Similarity and contagion-based thinking frequently underlies taboos, superstitions, folk magic and folk theories about the way the world works. For instance, to curse someone, one might obtain a lock of the target’s hair and ceremoniously defile it. Because the hair is from the person and even grew from them, it is thought to retain that person’s essence after separation. What befalls the hair will befall the hair’s owner. This principle is most blatantly symbolized through voodoo dolls, but more subtle variations of similarity and contagion exist in cultures worldwide. Similarity and contagion can be positive as well as negative – if we get hold of a guitar pick used by a rock star, we might think it will boost our own playing ability.
In one experiment, graduate students refused to drink tea that had been stirred with a sanitized flyswatter, even when offered money to do so. This and similar experiments and observations show that similarity and contagion-based thinking is not just a cultural holdover from more primitive times, but an ingrained aspect of human cognitive processing that is coded into our genes. Similarity and contagion-based theories are heuristics – that is, rules of thumb – which helped our ancestors survive in the harsh world in which our species evolved. Most of the time, these heuristics were useful – for instance, it really is stupid to consume a fresh-looking piece of food if it is found in a pile of rotting food. But under different conditions, especially modern conditions, these heuristics no longer apply. Similarity and contagion-based thinking is not necessary scientific, and in many cases can get in the way of science.